1. Field of the Invention
This invention relates to hermetic sealing of two silicon wafers.
2. Prior Art
U.S. Pat. No. 4,415,948 teaches bonding of two highly doped silicon wafers, one with an etched cavity, together by using an intermediary glass coating deposited on one wafer. The other wafer is sealed to the coated wafer by electrostatic bonding. In this device, the silicon itself acts as an electrical conductor and no feedthroughs are used. This patent neither teaches nor suggests the special fabrication features which can be advantageously used when lightly doped silicon wafers are used and when contact to metalized electrodes on the inner surfaces of an enclosed cavity between the silicon wafers is made by a highly conductive path in the lightly doped silicon.
U.S. Pat. No. 4,420,790 teaches forming a cavity by putting silicon spacers between two silicon plates that are covered with passivation layers of silicon dioxide and by a combination of silicon dioxide and silicon nitride layers. These layers electrically isolate the capacitor electrodes. The sensor cavity is not hermetically sealed but is open to the ambient. The two silicon plates are joined together by solder bumps and the spacing between the plates is kept constant by the spacers.
U.S. Pat. No. 4,424,713 teaches making a sensor structure including plates of glass, silicon and glass. The silicon is sealed to the glass by electrostatic bonding. The silicon plate has cavities on both sides, the cavity on one side being bigger than on the other side in order to provide stress relief on the silicon diaphragm. The teachings of this patent include relieving stresses at the glass silicon interface where the silicon diaphragm of a pressure sensor is bonded to glass plate by anodic bonding on both its front and back surfaces. Contact to electrodes on plate glass is made by metalized holes in the glass plate.
U.S. Pat. No. 4,390,925 teaches making a sensor with multiple interconnected cavities in a silicon plate bonded electrostatically to a glass plate. Such a structure is taught to be a high pressure sensor.
U.S. Pat. No. 4,184,189 teaches making a sensor with two metalized glass plates bonded together by a sealing glass mixture at about 500.degree. C. The spacing between plates is achieved by shim stock, such as a glass sealing spacer ring.
U.S. Pat. No. 4,207,604 teaches making a pressure sensor with a pair of insulating metalized plates sealed by melting glass frit. A metalized guard ring for the capacitor is used.
U.S. Pat. No. 4,345,299 teaches using two metalized ceramic substrates with an annular sealing ring to form a capacitive pressure transducer.
U.S. Pat. No. 4,177,496 teaches metalizing a pair of thin insulating plates (alumina quartz, pyrex) to form a capacitor. Glass frit or ceramic base material is fired to form a seal.
U.S. Pat. No. 4,261,086 teaches making a sensor by using a silicon wafer with etched cavities and anodically bonding it to a relatively thick glass substrate, for example, 1 mm to 2 mm. The capacitor electrodes are formed by metalizing the glass and highly doping the silicon cavity surface. Electrical connections are made through metalized holes in the glass.
U.S. Pat. No. 4,386,453 teaches making a sensor by using a silicon wafer with etched cavities anodically bonded to a glass substrate. There is metallization of holes drilled in the glass. The metalized holes are used as electrical feedthroughs to make contact to capacitor plates.
Even though there are known ways of coupling two wafers of silicon, there still remains a need for a method of sealing two wafers together which provides a reliable hermetic seal and which can be used at relatively low temperatures so as not to adversely affect electronic circuits which may be on the same piece of silicon. That is, while the application at high temperatures may provide a good seal, such high temperatures may adversely affect the operation or structure of electronic circuits also exposed to such temperature. These are some of the problems this invention overcomes.